Generally, this invention concerns radio communications system and more particularly cellular radio communications systems which use radio frequency (RF) capsules within a standard cell.
Traditional cellular systems break down a geographic area into groupings of cells. Each cell provides communication services to mobile stations contained within that cell. In order to provide these services, a base station provides a broadcast control channel upon a predetermined frequency from which all mobile stations contained within the cell can listen. In response, a mobile station can transmit a random access channel burst back to the serving base site whereupon the base site allocates a traffic channel for which the mobile station may communicate to the base site for providing communication services such as voice and or data services.
There is an inherent limitation on the number of mobile stations to which a base site may provide communication services. This limitation is determined by the cell size, the frequency reuse pattern of the cell and its surrounding cells and the number of frequencies or traffic channels allocated to the cellular system. In order to increase the capacity provided by a cellular system the physical size of the cells may be reduced, thus providing more cells within a particular system.
As cellular systems become more prevalent in our society and the usage of the systems increases, there is a need to provide more capacity within the cellular systems.
When implementing cellular systems within large buildings there are many sources of interference and also a large concentration of people that could use the system.
Traditional systems provide many extremely small cells to provide service to all the users within the building. By providing this plurality of very small cells, there is often a need to hand over more frequently between the cells creating a large amount of overhead communications between cells. Examples of such handover procedures may be found in the Global System for Mobile communications (GSM) recommendations.
In a conventional GSM system, when a mobile station (MS) or handset starts to move from the area of coverage of one cell to that of another, the MS makes measurements of the surrounding broadcast control channels (BCCH""s) contained in the broadcast allocation (BA) list. The instruction to measure the surrounding BCCH""s is received from the base station subsystem (BSS), which also receives the measurements made by the MS and uses these measurements to decide whether or not to switch between cells and also which cell is the most appropriate to switch to. This is known as mobile assisted handover.
Additionally, these systems are extremely sensitive to interference caused by adjacent cells, consequently they are fixed geographically and must be re-planned for each change in the dynamics of the system.
When considering a traditional cellular communications system, were the physical size of the cells contained therein to shrink to a few meters in diameter, the complexity of handing over calls between a first cell and a second cell would arise more often, and thus the complexity of the handover procedure would increase thereby requiring a more cumbersome system architecture.
In order to reduce the handover overheads and to improve the reuse factor of a conventional GSM cellular communications system, capsule based systems have been used. A capsule based system comprises a number of radio frequency capsules within the geographic area of a cell which services a high number of MS""s in a small area. The capsules use only the BCCH of that cell, there are not individual BCCH""s for each capsule. The BCCH is simucast for all capsules within the cell, i.e. the same information is being transmitted for each capsule. As such, the capsule controller has no information about the capsules available to assist in the handover process.
Additionally, cell overlap creates interference problems which are immeasurable by the conventional system and therefore discouraged. Thus, in order to design properly such a small system, the placement of the cells must be carefully planned because any changes such as office reconfiguration, tearing down walls or just moving furniture within the offices can create enough overlap for which a cellular re-plan is required, thereby creating a large amount of headache for system implementers.
The problems outlined above were addressed in U.S. Pat. No. 6,064,661, which patent is assigned to the assignee of the present invention. Here it was suggested that a separate measurement receiver be built in to each RF capsule. The controller is then able to request for a measurement of signal strength to be made on a given time slot and radio frequency. This enables the controller to make decisions about whether a call should be handed from one capsule to another. However, the addition of a further receiver and the associated processing add a large overhead to the complexity and cost of such a system. As such, there is a requirement that the above highlighted deficiencies be addressed in such a way as to reduce the complexity and cost of such communications systems.
The present invention aims to address all or some of the above disadvantages.
In accordance with a preferred embodiment of the present invention there is provided a method of assessing a signal in a second capsule within a cell of the cellular communications system, wherein a measurement indicative of the signal is made by a first capsule whilst it is servicing a call.
This method has the advantage of allowing a capsule controller in a picocellular or microcellular (or other) communications system to establish the strength of a signal of an adjacent capsule whilst enabling a decision to be made about the most suitable capsule to carry a call from a mobile station. This assessment procedure allows smooth handovers between capsules in such systems where there is no provision for mobile assisted handover, because all the relevant capsules reside within a single cell with a corresponding single BCCH.
The present invention also provides a cellular communications system comprising at least one cell, at least one capsule designated therein, and a capsule controller, wherein a measurement indicative of a signal in a second capsule is made by a first capsule whilst it is servicing a call. This system reflects the same advantages of the above method and further allows the use of the same frequencies in a cell at the same time. This is facilitated by the use of different capsules, thus providing for a greater number of MS""s to be supported by the cell.
Additional specific advantages of the present invention are apparent from the following description and figures.